ASoC: sun4i-codec: Add "Right Mixer" to "Line Out Mono Diff." route

The mono differential output for "Line Out" downmixes the stereo audio
from the mixer, instead of just taking the left channel.

Add a route from the "Right Mixer" to "Line Out Source Playback Route"
through the "Mono Differential" path, so DAPM doesn't shut down
everything if the left channel is muted.

Fixes: 0f909f98 ("ASoC: sun4i-codec: Add support for A31 Line Out
		      playback")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>